1. Field of the Invention
The present invention relates to a mechanically and/or electrochemically grained and optionally anodized base material composed of aluminum or its alloys. The base material includes a hydrophilic layer composed of at least one polymer containing basic and acidic groups. The base material can be used as a radiation-sensitive recording material having the base and a radiation-sensitive layer, from which offset printing plates can be produced.
2. Description of Related Art
Base materials known in the art for offset printing plates are provided with a photosensitive layer (copying layer), with whose aid a printing image is generated photomechanically. After the production of the printing image, the base layer carries the printing image areas and at the same time forms the hydrophilic image background, for the lithographic printing process, at the image-free areas-(non-image areas).
Suitable base materials for such base layers are metals such as aluminum, steel, copper, brass or zinc. Plastic sheets or paper are also suitable. In the printing-plate field, aluminum and its alloys have gained acceptance as substrates for base layers. The surface of the aluminum or aluminum alloy is grained mechanically, chemically and/or electrochemically by known methods and optionally anodized. Such pretreatments are however, not sufficient for base layers, which must meet the following requirements:
After the exposure, relatively soluble parts of the photosensitive layer must be removable from the base easily and without residue during the development in order to generate the hydrophilic non-image areas. Any residues of the layer still adhering to the base are recognizable as color haze since photosensitive layers are generally intensively colored. The consequence thereof is that the printing plate may "scum" at these points. PA1 After the exposure and development, portions of the non-image areas of the printing plates frequently still have to be corrected, with undesirable image components being stripped. The non-image areas laid bare in this process should not differ in color and lightness from the non-image areas laid bare by the developer. The uniform lightness is necessary in order to be able to use measuring instruments with which the proportion of the area of the image regions is determined by means of the lightness difference between image regions and non-image regions. PA1 The undesirable lightness difference between a non-image area produced by correction and one produced during the normal development process is designated as correction contrast. PA1 The base laid bare in the non-image areas must be sufficiently hydrophilic in order to take up water rapidly and permanently during the litho-graphic printing process. Water is what repels the greasy printing ink. PA1 The photosensitive layer must not peel from the base material before the exposure, and the printing part of the layer must not peel from it after the exposure. PA1 have very good hydrophilizing properties, PA1 are equally suitable for all photosensitive layers without the photosensitive layer being impaired by reaction with the hydrophilizing agent on prolonged storage, and PA1 have a very good adhesion to the printing areas of the layer.
Normally, the base material is additionally hydrophilized because it does not otherwise absorb sufficient water. The hydrophilizing agent must be matched to the particular photosensitive layer in order to avoid undesirable reactions and impairment of adhesion.
The known hydrophilizing methods are (regardless of the photosensitive layer, the developer solution or the correcting fluids) subject to more or less considerable disadvantages. For example, after the treatment with hydrophilizing alkali-metal silicates, which result in a good developability and hydrophilicity, an impairment of the photosensitive layers has to be accepted after prolonged storage time.
If the base materials are hydrophilized with water-soluble polymers, their good solubility (particularly in aqueous alkali developers such as those predominantly used for the development of positive-working layers) results in a marked reduction in the hydrophilizing action. In the case of polymers containing sulfonic acid groups, the interaction of the free anionic acid functional groups with the diazo cations of negative-working photosensitive layers manifests itself adversely. The result is that, after development, a marked color haze due to retained diazo compounds is recognizable on the non-image areas. Polymeric acrylic acid derivatives are disadvantageous because, in an application form in which they are able to prevent color haze, i.e., in a solution of 0.1 to 10 g/l, they are very viscous and an excess can only be removed from the surface of the base with considerable efforts. Particularly susceptible to color haze formation are highly photosensitive layers which are used for imprinting with lasers (EP-A 0 364 735) and which contain a polymeric binder, a free-radical-polymerizable compound containing at least one polymerizable group and a photoreducable dye, a radiation-cleavable trihalomethyl compound and a metallocene compound as photoinitiators. Particularly high requirements are therefore imposed on the hydrophilic base material so that no constituents of the photosensitive layer remain behind on the non-image areas.
From DE-C 11 34 093 (equivalent to U.S. Pat. No. 3,276,868) and U.S. Pat. No. 4,153,461, it is known to hydrophilize the base material with phosphonic acids, in particular with polyvinylphosphonic acid or copolymers of vinylphosphonic acid with acrylic acid and vinyl acetate. It is also mentioned that salts of the phosphonic acids are suitable. This is not, however, specified in greater detail.
EP-A 0 069 320 (equivalent to U.S. Pat. No. 4,427,765) discloses a method of hydrophilizing an aluminum base material for planographic printing plates in which salts of polyvinylphosphonic acids, polyvinyl- sulfonic acids, polyvinylmethylphosphinic acids and other polyvinyl compounds containing at least divalent metal cations are used.
According to EP-A 0 190 643, the base material is coated with a homopolymer of acrylamidoisobutylene-phosphonic acid or a copolymer of acrylamidoisobutylene-phosphonic acid and acrylamide or with a salt of said homopolymer or copolymer containing an at least divalent metal cation. The coating has the advantage that the finished printing plates exhibit a good hydrophilicity at the non-image points and have a reduced color haze.
EP-A 0 490 231 describes the treatment of printing-plate bases with polyethylenimines which contain structural elements of the type --(CH.sub.2 --CH.sub.2 --N(X)--).sub.n -- or with polyvinylamines which contain structural elements of the type --(CH.sub.2 --CH(NY.sup.1 Y.sup.2)--).sup.n --, X, Y.sup.1 and Y.sup.2 being optionally C-substituted sulfomethyl groups or phosphonomethyl groups. However, satisfactory results are still not achieved with this method.